Release layers facilitate removal of a negative or positive image-bearing film from a flexographic printing element after the element has been imagewise exposed through the negative or positive film.
U.S. Pat. No. 4,576,897 discloses applying a release layer to an alcohol-soluble or water-soluble photosensitive polyamide composition wherein the release layer comprises a polymer which is soluble or dispersible in a developer for the photosensitive polyamide composition. Polyvinyl alcohol is mentioned as a suitable component for a release layer to be applied to a water-soluble photosensitive polyamide composition. No other additives, such as a plasticizer, are used in preparing the release layer. In fact, it is mentioned specifically that the reason the release layer works so well is that such additives are not used.
Release layers can be applied in a variety of ways. For example, the release layer can be coated onto a cover sheet which can then be laminated on the surface of the photosensitive printing element. Another option is to extrude the photosensitive composition between a cover sheet containing the release layer and a support. The release layer remains on the surface of the photosensitive composition when the cover sheet is removed.
The problem with a polyvinyl alcohol release layer, such as that taught above, is that it wrinkles when the cover sheet is removed. These wrinkles are visible on the surface and appear as grooves in the resulting image after the photosensitive element has been imagewise exposed and developed.
It is believed that no one heretofore has produced an aqueous or semi-aqueous processible release layer which does not wrinkle on the surface of the photosensitive composition of an aqueous or semi-aqueous processible flexographic printing element after the cover sheet has been removed.